This invention relates to a photothermographic material and more particularly, to a photothermographic material suited for the manufacture of printing plates.
Nowadays, it is strongly desired in the printing art from the standpoints of environmental protection and space saving to reduce the amount of waste solution used in the processing of conventional photographic photosensitive materials. There is a need for the technology relating to photothermographic photosensitive material for graphic printing application which can be effectively exposed by means of a laser image setter and form a clear black image having high resolution and sharpness. The photothermographic photosensitive material can provide the customer with a thermographic process system which eliminates a need for solution system processing chemicals and is thus more simple and environment friendly.
Photothermographic materials which are processed by a photothermographic process to form photographic images are disclosed, for example, in U.S. Pat. Nos. 3,152,904 and 3,457,075, D. Morgan and B. Shely, "Thermally Processed Silver Systems" in "Imaging Processes and Materials," Neblette, 8th Ed., Sturge, V. Walworth and A. Shepp Ed., page 2, 1969.
These photothermographic materials generally contain a reducible silver source (e.g., organic silver salt), a catalytic amount of a photocatalyst (e.g., silver halide), a toner for controlling the tonality of silver, and a reducing agent, typically dispersed in a binder matrix. Photothermographic materials are stable at room temperature. When they are heated at an elevated temperature (e.g., 80.degree. C. or higher) after exposure, redox reaction takes place between the reducible silver source (functioning as an oxidizing agent) and the developing agent to form silver. This redox reaction is promoted by the catalysis of a latent image produced by exposure. Silver formed by reaction of the organic silver salt in exposed regions provides black images in contrast to unexposed regions, forming an image.
Such photothermographic materials have been used as microphotographic and medical photosensitive materials. However, only a few have been used as a graphic printing photosensitive material because the image quality is poor for the printing purpose as demonstrated by low maximum density (Dmax) and soft gradation.
With the recent advance of lasers and light-emitting diodes, scanners and image setters having an oscillation wavelength of 600 to 800 nm find widespread use. There is a strong desire to have a high contrast photosensitive material which has so high sensitivity and Dmax that it may comply with such output devices. A demand for a simple dry process is also increasing.
U.S. Pat. No. 3,667,958 describes that a photothermographic material comprising a polyhydroxybenzene combined with a hydroxylamine, reductone or hydrazine has high image quality discrimination and resolving power. It was found that this combination of reducing agents is likely to increase fog. U.S. Pat. No. 5,496,695 discloses a photothermographic material comprising an organic silver salt, a silver halide, a hindered phenol and a certain hydrazine derivative. Undesirably, these hydrazine derivatives fail to provide a fully satisfactory maximum ultimate density or ultrahigh contrast and allow black pepper to generate, resulting in poor image quality.
A photothermographic material having high Dmax and high contrast can be obtained by adding a hydrazine derivative to a photosensitive material as disclosed in Japanese Patent Application No. 228627/1995. This photothermographic material has somewhat high Dmax and ultrahigh contrast, but allows black pepper to generate at dot edges, exacerbating image quality.